1021. Deepest Root (25) -并查集判树 -BFS求深度

题目如下：

A graph which is connected and acyclic can be considered a tree. The height of the tree depends on the selected root. Now you are supposed to find the root that results in a highest tree. Such a root is called
the deepest root.

Input Specification:

Each input file contains one test case. For each case, the first line contains a positive integer N (<=10000) which is the number of nodes, and hence the nodes are numbered from 1 to N. Then N-1 lines follow, each describes an edge by given the two adjacent
nodes' numbers.

Output Specification:

For each test case, print each of the deepest roots in a line. If such a root is not unique, print them in increasing order of their numbers. In case that the given graph is not a tree, print "Error: K components" where K is the number of connected components
in the graph.

Sample Input 1:

5
1 2
1 3
1 4
2 5

Sample Output 1:

3
4
5

Sample Input 2:

5
1 3
1 4
2 5
3 4

Sample Output 2:

Error: 2 components

这个题有N个顶点，N-1条边，属于稀疏图，应该用邻接矩阵而不是邻接表，否则会内存超限。

刚开始拿到这道题目的时候我想到的方法是用带颜色标记的BFS判断环路，并且排除掉，但是这样会超时，后来在网上查阅，看到大家都是使用并查集来判断是否是树，通过把所有元素归入并查集，并且在查询根的时候压缩路径，如果所有元素都是同根的，那么把他们加入Set后集合的规模为1，说明是一棵完整的树，如果不是如此，则说明图由多个连通的部分组成，连通部分的个数等于根的个数，因此只需要输出Set的规模即可。

如果是一棵树，只需要使用BFS来遍历树中的每一个顶点，来计算深度。

为了计算深度，改进BFS，加入三个变量：head、last、tail，last表示这一层的最后一个元素，tail表示下一层的某个元素，初始化时，让last=s，代表第一层只有源点，接下来在出队一个结点，并让head等于这个结点，然后在遍历结点的邻接点时不断的更新tail，不难看出tail在最后一次更新后指向的是下一层的最后一个元素，然后判断head是否等于last，如果是代表本层结束，把层计数变量+1，更新last为tail，以此类推即可。

#include <iostream>
#include <stdio.h>
#include <queue>
#include <set>

using namespace std;

#define MAX 10002

bool visited[MAX];
vector<vector<int> > Graph;
int N;
int P[MAX];

void initSet(int N){

    for(int i = 1; i <= N; i++) P[i] = i;

}

void CompressSet(int x, int top){

    if(P[x] != top){
        CompressSet(P[x],top);
        P[x] = top;
    }

}

int FindSet(int x){

    if(x != P[x]){
        int t = FindSet(P[x]);
        CompressSet(x,t);
    }
    return P[x];

}

void UnionSet(int x, int y){

    int p1 = FindSet(x);
    int p2 = FindSet(y);
    P[p1] = p2;

}

int BFS(int s){

    for(int i = 1; i <= N; i++) visited[i] = false;
    int head,tail,last;
    int stage = 0;
    queue<int> Q;
    Q.push(s);
    visited[s] = 1;
    last = s;
    while(!Q.empty()){
        int v = Q.front();
        visited[v] = 1;
        head = v;
        Q.pop();
        for(int index = 0; index < Graph[v].size(); index++){
            int w = Graph[v][index];
            if(!visited[w]){
                Q.push(w);
                tail = w;
            }
        }
        if(head == last){
            last = tail;
            stage++;
        }

    }

    return stage;

}

int main()
{
    cin >> N;
    int v1,v2;

    Graph.resize(N + 1);
    initSet(N);
    for(int i = 1; i <= N; i++){
        visited[i] = false;
    }

    for(int i = 1; i < N; i++){
        scanf("%d%d",&v1,&v2);
        Graph[v1].push_back(v2);
        Graph[v2].push_back(v1);
    }

    for(int i = 1; i <= N; i++){
        for(int j = 0; j < Graph[i].size(); j++){
            UnionSet(i,Graph[i][j]);
        }
    }

    set<int> root;
    for(int i = 1; i<= N; i++){
        root.insert(FindSet(i));
    }

    if(root.size() != 1){
        printf("Error: %d components\n",root.size());
        return 0;
    }

    vector<int> maxDeepNodes;
    maxDeepNodes.clear();
    int maxDepth = 0;
    int depth = 0;

    for(int i = 1; i <= N; i++){
        depth = BFS(i);
        if(depth > maxDepth){
            maxDepth = depth;
            maxDeepNodes.clear();
            maxDeepNodes.push_back(i);
        }else if(depth == maxDepth){
            maxDeepNodes.push_back(i);
        }
    }

    for(int i = 0; i < maxDeepNodes.size(); i++){
        printf("%d\n",maxDeepNodes[i]);
    }

    return 0;
}